The result gives Phoebe, from Chelmsford, a current ranking of fourth place in the UK in the 60m hurdles indoors at U13G level.
Year 7 student Phoebe, who is a member of the Chelmsford Athletics Club, and trains with the team weekly, said: "I really did not expect to win but I am really pleased, especially as I achieved a person best."
Sarah Bellamy, Head of Physical Education at Chelmsford County High School, said: "Phoebe trains with the school team on a regular basis.
''She shows true dedication and commitment for her chosen sporting activity. We wish her every success in her future events."
Phoebe also competed in the Eastern Athletics Association competition where she achieved second place in the hurdles and fourth place in the 60m sprint.